Signal transmission by guided and unguided waves



June 1931- R. R. ADAMS ET AL SIGNAL TRANSMISSION BY GUIDED AND UNGUIDED WAVES Filed Sept. 10, 1924' 5 Sheets-Sheet 1 R. R. ADAMS ET AL June 23, 1931.

SIGNAL TRANSMISSION BY GUIDED AND UNGUIDED WAVES Filed p 1924 5 Sheets-Sheet 2 June 23, 1931. R. R. ADAMS ET AL SIGNAL TRANSMISSION BY GUIDED AND UNGUIDED WAVES Fil p 1924 5 Sheets-Sheet 3 'zmrms June 23, 1931. R. R. ADAMS ET AL SIGNAL TRANSMISSION BY GUIDED AND UNGUIDED WAVES Filed Sept. 5 Sheets-Sheet 4 5 Sheets-Sheet 5 R. R. ADAMS ET AL Filed Sept. 10, 1924 SIGNAL TRANSMISSION BY GUIDED AND UNGUIDED WAVES June 23, 1931.

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Patented June 23, 1931 UNITED STATES PATENT OFFICE RAY R. ADAMS, 0F MAPLEWOOD. NEW JERSEY, AND ELMER V. GRIGG S, 0] WHITE PLAINS, NEW YORK, ASSIGNORS TO WESTERN ELECTRIC: COMPANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK SIGNAL TRANSMISSION BY GUIDED AND 'UNG-UIDED WAVES Application filed September 10, 1924. Serial No. 736,830.

This invention relates to toll and ointto-point radio transmission systems. ne of the outstanding characteristics of radio transmission is that the medium by which the waves are transferred from the point of origin to the receiving station is capable of transmitting waves of a very great frequency range with substantially negligible discrimination. This is in marked contrast 'to the transmission characteristics of certain ciraccomplished over submarine cables of even a few hundred miles length. With respect to the feature of privacy so far as insuring that the message is to be received only by those for whom it is intended, the advantage is with the conductive circuit. It is, of course, true that radio systems havev been developed which are highly secret to ordinary radio receivers, but they are, for the most part, very complex and expensive and they are subject to reception by an unauthorized receiving station disposed in the general direction of the maximum wave transmission and equipped with suitable receiving apparatus.

A number of situations may arise in whichit is essential to be able to transmit, without frequency discrimination, a broad band of waves representing speech, music or other intelligence, and at the same time, to restrict the reception of .the transmitted information to those for whom it is intended. For example, in radio broadcast transmission'of programs consisting of general information, market and-sports reports, announcements, proclamations, musical and other forms of entertainment, etc., the broadcasting'station is, at present, unable to restrict reception to any particular individuals and, accordingly, is unable to derive any direct revenue from those who receive and enjoy the programs.

An object of this invention is to provide a toll radio broadcasting system which will enable the reception of the intelligence broadcast to be restricted to those who are subscribers to the service.

In the field of transoceanic telephony, radio transmission has clearly demonstrated its practicability. In order, however, to make radio telephon entirel satisfactory for business and ot er con dential communication, its reception must be restricted to those for whom it is intended.

An object of the present invention is to provide a transoceanic telephone system combining high quality with privacy.

The broadcasting of a number of different programs simultaneously is ordinarily accomplished by use of an individual carrier wave for each of the programs, no two carrier waves being of the same frequency. It is, therefore, possible by controlling the tuning or selectivity at the receiving station to select one particular program to'the exclusion of those transmitted on carrier waves of some other frequency.

Another object of this invention is to enable a properly authorized receiving station to select any desired one of a number of programs being simultaneously broadcast and to insure that an unauthorized station will be unable to receive any of these programs.

Consideration of a few simple principles underlying carrier wave transmission will make clear the fundamental elements of this invention.

As is explained at page 308 of the article Carriercurrent telephony and telegraphy by Colpitts and Blackwell, Journal of the American Institute of Electrical Engineers, vol. 40, No. 4, April, 1921, a speech modulated carrier wave comprises an upper side band and a lower side band together with an unmodulated component of the carrier frequency. For audible reception of such a wave, it is necessary to transmit one of the side bands, but the other side band may be and preferably is suppressed. In order to til derive the desired message from the transmitted side band, it is necessary to cause it to interact with an unmodulated wave of the carrier frequenc If, therefore, a single side band be transmitted, the other side band and the unmodulated carrier frequency component being suppressed, it will be necessary at the receiving station to supply an unmodulated wave of the carrier frequency. Systems for accomplishing this are well known. See for example, Carson U. S. Patent No. 1,449,382, March 27, 1923, and Arnold U. S. Patent No. 1,449,372, March 27, 1923.

The intelligible reception of a single side band of a modulated carrier wave may, in 'eneral, be readily accomplished by providmg at the receiving station, a local source of waves of the carrier frequency. These locally produced waves are impressed together with the incoming side band upon the detector. Any difference in frequency between the locally produced oscillations and the unmodulated carrier wave at the transmitter will result in unintelligible or poor quality signals. However, it ispossible, particularly if an electron relay oscillator be used at the receiving station, to readily adjust its frequency during reception in order to bring it into substantial agreement with the frequency of the carrier wave source at the transmitting station. Accordingly, messages transmitted by a single side band may be readily received at any receiving station provided with a local oscillator.

In order to prevent reception by unauthorized persons of the message transmitted, it is proposed, in accordance with this invention, to utilize a Varying frequency carrier wave and to radiate a single side band of the resulting modulated carrier -wave. Since the intelligible reception of the single side band requires at the receiving station, that an unmodulated component of the carrier frequency be introduced, it follows that reception cannot be accomplished by the use of local oscillations of constant frequency.

If, however, unmodulated local oscillations be supplied which are of the same frequency as the carrier wave at the transmitting station and which undergo the same variations in frequency as that carrier, the result of the demodulating action at the receiver will be to give the desired signal. Such varying frequency local oscillations may be supplied directly over a conductive circuit either as oscillations of the carrier frequency or as oscillations of some sub-multiple frequency from which carrier frequency oscillations may be derived.

Of course, it is possible from a theoretical standpoint to transmit over the conductive circuit the whole signal Wave bothside band and carrier, thereby dispensing with the radiated element. But a distinct advantage is gained by employing both radiation and conduction. The varying carrier transmitted over wires varies in he uency over comparatively narrow limits, a out 2000 cycles and the variations are slow, whereas for first class reception such .as is-now' required b I uct of the amplitudes of the carrier and side band.

The novel features which are considered characteristic of the invention are pointed out with particularity in the appended claims. The invention itself, however, both as to the details of its organization and its mode of operation will be best understood by. reference to the following description taken inconnection with the accompanying drawings inwhich Fig. 1 illustrates a circuit diagram of a radio station for broadcasting information or other programs to subscribers stations associated with a telephone exchange. Fig. 2 shows circuits of a telephone exchange together with its toll subscriber circuit connections which may cooperate with the broadcasting station of Fig. 1. Figs. 3 and 4.- illustrate respectively, the transmitting and receiving apparatus of a transoceanic telephone system. Fig. 5 illustrates diagrammatically a modification of the system of'Figs. land 2 in which any subscribers station may select a desired program from among those which are being broadcast.

In Fig. 1 there is shown a broadcasting stationcomprising a circuit 1 including a microphone 2 or other pick-up device in series with a source of current and the primary of a transformer this circuit serving 1- as the origin of the speech or program currents. The pick-up vdevice may be located in a local studio or may be at the remote end of a long telephone line terminating at the broadcasting station. The program currents are supplied by the transformer to a low pass filter 3 having a transmission ran e which may, in the case of ordinary speech, be limited tofrequencies less than 2000 cycles, but which for musical programs, preferably extends to an upper limit of the order of 5000 to 10000 cycles. A vacuum tube oscillator .4 of well-known type supplies currents of, for example, 20,000 cycles frequency to a one-way conducting amplifier hit . monies.

5 of the well-known electron relay type from the output circuit of which the currents are impressed by a transformer 6, upon the common input path of a balanced vacuum tubemodulator 7, also of well-known type. If the oscillations from source 4 are assumed to be of 20,000 cycles frequency and the program currents passed by filter 3 are of 0 to 2 000 cycles, the modulated currents in the output circuit of the modulator will include two side bands of 18,000 to 20,000 and 20,- 000 to 22,000 cycles respectively. By means of a high pass filter 8 of well-known type (see Campbell Patent No. 1,227 ,113, May 22, 1917, and page 310 of the article by. Colpitts and Blackwell), the upper side band may be selected to the exclusion of the lower. It will, of course, be understood that it is not necessary to use the precise frequencies indicated, that a band filter may be used instead of a high pass filter, and that, if desired, a low pass filter may be used to select the lower side band to the exclusion of the upper. The side band selected by the filter 8 is impressed upon the second balanced modulator 9, similar in character to modulator 7 and is used to modulate high frequency carrier oscillations, supplied by a vacuum tube oscillator 10 through an electron relay 11, and a transformer associating its output circuit with the common input path of thebalanced modulator 9. Oscillator 10 has an oscillatory circuit consisting of condenser C variable condenser C and inductance L. The condenser C has a fixed plate 12 and an irregular plate 13 carried by the constantly rotating shaft of a motor 14. Thus the capacity of the condenser C is varied in an irregular manner. It follows that the oscillator 10 will supply to amplifier 11 and balanced modulator 9, oscillations which vary in frequency in an irregular manner. The plate 13 may be of various shapes depending on practical considerations such as the range and speed of the variations introduced into the carrier. It may also be possible to replace plate 13by plates of other types so that the sequence in variations in the carrier could be changed from time to time. Let it be assumed for example, that the mean frequency of the oscillator 10 is 50,000 cycles and that its frequency ranges from a minimum of 48,000 to a maximum of 52,000 cycles. From the output of the balanced modulator, a high pass filter 15 selects waves of frequencies above 60,000 cycles, thus including the upper side band resulting from the modulation of the varying frequency carrier supplied from the output of relay 11 by the band of waves supplied by filter 8 to the input circuit of modulator 9. Obviously, element 15 may be a band filter if it is desirable to suppress objectionable har- An amplifier 16 which may provide any desired number of sta es of am lification with additional selectivity, ampli es the selected side band and supplies 1t to the radiating antenna 17.

The purpose of the oscillator 4 in producing oscillations of 20,000 cycles is to enable the original signalingcurrents of circuit, 1 to be stepped up in frequency before being used to modulate the variable carrier waves supplied by source 10., This enables the side bands produced in the output circuit of modulator 9 to be separated sufiiciently in,

frequency from the variable carrier wave to enable filters to select one side band and exclude the other. Moreover, it enables the filter 15 to aid the balanced modulator 9 in suppressing any residual unmodulated component of the variable carrier frequency which might ensue as a result of imperfect balance in the modulator. What is trans mitted by antenna 17 maybe considered.

either as a carrier wave of 50,000 cycles frequency having variations from 48,000 to 52,000 cycles, and modulated by a band of from 20,000 to 22,000 cycles frequency, or as a variable carrier wave of 70,000 cycles mean frequency modulated by s eech. Since the wave radiated by antenna 1 consists only of ones'ide band, it is essential in order to receive it, that an unmodulated component be supplied at the receiver. Since the frequency of this unmodulated component to be supplied at the receiver must vary irregularly in consonance with that of source 10, there is nothingradiated which will serve to enable an unauthorized receiving station to intercept the message.

In order to supply an irregular variable frequency component. which will enable subscribers to the service to pick up the message, oscillations from oscillator 4 are impressed through an amplifier 18 to balanced modulator 19 to which oscillations are also supplied by source 10 through an electron relay 20. High pass filter 21 selects the upper side band of the resulting modulated wave and this side band will extend from 68,000 to 72,000 cycles with a mean frequency of 70,000 cycles. This upper side band is transmitted to a conductive circuit 22 which may be a toll line or special circuit leading to a telephone exchange. In the case of a toll line, it is desirable to insert condensers 23 of such capacity as to pass high frequency current with little impedance, while substantially blocking ordinary telephone currents transmitted over the toll line 22.

Referring to Fig. 2, the toll line 22 is indicated with a high frequency termination vided with condensers 26 of such capacity as to pass the .high frequency current with little impedance while substantially blocking ordinary currents transmitted over the toll line 22. The low frequency termination 24 is to enable the use of the toll line 22 for ordinary telephone purposes. These terminations are at a central station. at which similar toll lines 27 with corresponding terminations are terminated. A patching cord 28 is provided for connecting a high frequency bus bar 29 to the toll circuit 22, an amplifier 30 being interposed in this connection if desired. It will be understood that as many other bus bar circuits similar to 29 may be employed as there are different programs to be transmitted. A subscribers line 31 leads to an ordinary subscribers station 32. This line may be connected with the toll line 22 by means of a patching cord 33. The line 31 is connected to the bus' bar 29 through condensers 34 of a proper value to freely pass the high frequency currents but substantiall block telephone cur rent. Connected to t e line 31 at the subscribers station is a high frequency path 35 by which the high frequency currents from the bus bar 29 are supplied to the detector 36 of the program set. An antenna 53 of suitable form, preferably a tuned loop, is also connected to the detector 36 in well known manner. Accordingly, there will be supplied to the receiver 38, low frequency signals or program currents corresponding to those of circuit 1 at the transmitting station. The bperation of the detector 36 in combining the side band radiated by antenna 17 and the unmodulated carrier component received over toll circuit 22 is too well known to require description. If desired, an amplifier 39 and loud speaker 40 may be connected to the output of the detector 36. To separate the high frequency transmission over the. subscribers line 31, from the ordinary tele hone conversations, condensers 41 and cholie coils 42 may be employed as shown.

From the central station, it may be desirable to transmit high. frequency currents received over circuit 22 to distant central stations, such as central station 43. This is accomplished by means of a toll circuit amplifier 44, toll terminal connections 45, patching cord 46, toll circuit termination 49, and a toll circuit. 47 extending to the central station 43. Similar toll circuits having terminations 48 extending to other exchanges ma also be connected to the output of amp ifier 44 by multiple connections similar to 45. At the exchange 43 is provided a high frequency circuit 52 which may be connected with the toll line 47 through similar high frequency terminations and which may also be connected to a subscribers line 49. A radio receiving circuit 50 is connected to the subscribers line in the same general manner as described in connection with the circuit 36. This receiving circuit differs from that previously described in that an ordinary crystal detector circuit is utilized. At the exchange 43, there may be provided as many high frequency circuits 52 as there are rograms being broadcast. Also at this exc ange, there are provided additional toll lines 51 from other distant stations.

Obviously, a subscriber having regular service for any particular program may have his circuit strapped as in circuit 31 to the high frequency circuit at the central station so that the program is available to him without calling the operator. If, however, he is to be furnished with any one of a number of programs upon request, the operator will be equip ed with high frequency connecting cord circuits as in the case of the subscribers line 49.

The fact that repeaters may be used for conductive circuits may be taken advantage of to extend the system of Figures 1 and 2 over large areas. It would appear, for example, that with the central broadcasting station at St. Louis or Chicago, it would be possible to cover the entire country with standard service.

The load on any particular supply circuit should be made as nearlyconstant as possible. This may be done by arranging the connecting circuits with resistance shunts of approximately the same resistance as lines which are to be connected. If, for example, in the circuits of Fig. 2, the amplifier 44 is to supply service to ten outgoing trunks 47, 48, etc., there may be times when certain of these trunks will be disconnected at the connections 45. At such times, these connections should have resistance shunts simulating the impedance in the trunks. In this manner, the volume which must be de livered from the amplifier 44 may be kept at substantially constant levels.

While the general idea of dividing the wave into two parts and transmitting over the wires the carrier current and broadcasting a modulated side band has been shown above, the arrangement of Figs. 1 and 2 is capable of perhaps more practical application in a modified form 1n which not the carrier current but a control current is transmitted over wires. This control current, of varyin frequency, has a very much lower mean requency than the carrier itself, but from it the varying carrier frequency can be derived at the receiving station or at a local distributing station. Transoceanic communication over submarine cables may be effected by the application of the foregoing rinciple. Since a cable used for transat antic service is capable of passing only a very limited range of low frequency withadvisable to transmit over the cable currents of very low frequencies.

In ig. 3 a varying frequency oscillator 60, similar to oscillator 10 of Fig. 1 delivers varying frequency current ranging from 15.5 to 17 cycles. This current is supplied successively to an amplifier 61, low pass filter 62 and three harmonic generators 63, 64, and 65. Each of the harmonic generators will preferably be designed to accentuate a particular harmonic of the current supplied it, as for example, a tenth multiple harmonic. Although not specifically illustrated, amplifiers may be supplied between successive harmonic generators and preferably band filters, as the band filter 66, shown between the last two of these harmonic generators, but of the appropriate transmission ranges, are connected between successive harmonic generators in each instance. There are now well known in the art, harmonic generators which will produce even harmonics only to the exclusion of odd harmonics or odd harmonics to the exclusion of even harmonics. Harmonic generators of this type are preferable in the present instance since in producing the 10th multiple harmonic, it will be only necessary for the succeeding band filter to discriminate against 8th and 12th harmonics whereas with the usual type of harmonic generator, suppression of the 9th and 11th harmonics will be required. The var ing frequency carrier wave of 15,500 to 1 ,000 cycles is thus sup plied to circuit 67, through band filter 68, and is impressed upon the input circuit of the balanced modulator 69.

A microphone or other telephone circuit 70 is inductively connected to filter 71 the transmission range of which is preferably from 200 to 2000 cycles. The output from the filter 71 is im ressed upon the input circuit of a balance modulator 72. A source 73 of alternating current. impresses upon the input circuit of the balanced modulator 72 a current of approximately 10,000 cycle oscillations which is modulated by the band transmitted by the filter 71. The output current of the modulator 72 is impressed upon a band filter 74 having a transmission range from approximately 10,100 to 12,500 and the output circuit of the filter is lnductively connected to the input circuit of the balanced modulator 69. Because of the transmission range of the filter 74 only the upper side band of the resultant current from the modulator 72 is transmitted to the modulator 69. The band of frequencies impressed upon the modulator 69 from the filter 74 modulates the waves impressed upon the modulator from the harmonic generator 65. The output circuit of the modulator 69 is inductively connected to a band filter 75 the transmission range of which is preferably from 25,000 to 30,000 cycles and serves to se arate the upper side band of the modulate waves from the modulator 69 from the lower side band thereof. The band transmitted by the filter 75 is impressed upon the input circuit of a modulator 76 by means of an inductive coupling. A 10,000 cycle wave is also impressed upon the input circuit of the modulator 76 from the oscillator 7 3, through an amplifier 77. 'The output circuit of the modulator 76 is inductively connected to a band filter 78 having a transmission range extendin from 15,000 to 20,000 cycles which in turn is inductively connected to an antenna 79. The filter 78 transmits the lower side band only of the varying frequency carrier Wave and effectively suppresses the other side band and any unmodulated carrier component which might be present if the modulators 69 and 72 were not perfectly balanced.

The low base frequency oscillator also supplies varying base frequency oscillations directly to the cable 80 through amplifier 81 and low pass filter 82. These are received at the receiving terminal and are impressed upon an attenuation equalizer 83, Fig. 4 which corrects for the different attenuations experienced by the different frequencies in transmission over the cable system. After the oscillations are equalized they are amplified by an amplifier 84 and are supplied to a series of harmonic generators 85, 86 and 87 which may be similar to generators 63, 64 and after which the desired harmonic is selected by a band pass filter 88 and impressed through a circuit 89 on the vacuum tube detector 90.

The particular harmonic generating apparatus at the receiver may well correspond to that used at the transmitter. Any number of stages may be used and the same considerations as to amplifiers and intermediate filters will apply. Radiated energy from the antenna 79 is received directly by a loop 91 or other radiation receiver and impressed upon the detector 90, the output circuit of which supplies either an amplifier loudspeaker arrangement 92 or a head receiver 93 through a low pass filter 94.

As there may possibly be a slight time lag in the arrival of the unmodulated band by way of the cable 80 and circuit 89 at the detector 90 with respect to the arrival of the modulated component radiated to the loop 91, an intermediate circuit 95 may be supplied between loop 91 and the detector 90 to regulate the phase of the energy received by the loop before impressing it on the detector90.

The series variable resistance shown in the network 95 is preferably of such an order as to prevent the tuning of the loop 91 being primarily dependent upon either the condenser 96 or the small variable condenser The application of this invention to the selection by subscribers of programs broadcast over toll line is illustrated in Fig '5. This figure has been considerablysimplified in order to facilitate the understanding of the part of the invention about to be described. It is to be understood, however,

that there may be incorporated in the cir-- cuits herein disclosed arrangements for generating varying currents similar to those described above in connection with- Figs. 1,

2,3and4.

.A broadcasting station 101, similar to that shown in Fig. 1, having an antenna 102, supplies both the antenna and the toll line for transmitting the unmodulated carrier wave as in the case shown in Fig. 1. Amplifier 53, which amplifies the unmodulated carrier wave corresponding to the wave transmitted through filter 21 to Fig. 1, has no counterpart in the circuit illustrated by that figure, although a similar amplifier may equally well be used therein. The unmodulated waves are supplied over a toll line 105 which is one of a group of toll lines 105, 106 and 107 terminating at a central station100 of a telephone system, each of these lines corresponding to the toll line 22 of Fig. 1. The terminations for ordinary speech currents are omitted in this figure since they have been indicated sufficiently in Fig. 2. Trunk circuits 108, 109 and 110 are connected respectively to circuits 105, 106 and 107 through band-pass filters '111, 112 and 113 whoa: function will be more full described hereinafter. Suitable automatically controlled switches may be provided for these connections or they may be made manually by means of plugs and jacks. The trunk circuits 108, 109 and 110 together with circuit 132 are multipled in a well known manner to selector or switches 118, which are individual to each subscribers line.

The subscriber is thus ermitted to select any desired program. witch 118 is controlled in the followingmanner: A source 130 located at the central exchange provides continuously a high frequency current of 8000 cycles and is permanently'connected to each of the subscribers lines through selective arrangements, such as choke coils 119, or simple tuned circuits designed to exclude speech and also to exclude currents of a carrier frequency range. These connections serve to maintain on the subscribers line control oscillations of 8000 cycles frequency. Condensers 145 in the subscribers line exelude control frequency from the trunk circuits, but permit the carrier currents of channels 105, 106 and 107 and the high fre- (f uenc currents from source 155, to be reerre to later, to pass readily to the subscribers lin'e. Accordingl the circuit for the control currents can e ectively' be completed only through the arm 133 and contacts 124 of the dial switch 123, which will hereinafter be described.

Associated with each subscribers control circuit is a'detecting arrangement 131 employin a grid leak blocking condenser. The subscri ers line is normally closed for control frequency currents thru a path including choke coils 119 and the primary of input transformer 146 and, therefore, these currents will be impressed upon detector 131, the grid leak circuit of which maintains the space current of this detector at substantially zero value. Through a resistance 114 this detector is connected to an amplifier 116; upon interruption of the 8000 cycle control oscillations, with consequent increase of space current through resistance 114, there will be a very sudden change of space current in the output of the amplifier 116 thus stepping the armature of the selector magnet 117. This selector magnet controls a selector switch 118 of a type well known in the machine switching telephone art. The selector switch 118 serves to connect a subscribers line to any desired high frequency trunk 108, 109 or 110.

A subscribers station is shown diagrammatically at 125. lVithin the square 126 is contained the ordinary telephone transmitter and receiver and the associated circuits. A calling dial for ordinary automatic telephony is indicated at 127 Coils 122 prevent interference of ordinary telephone conversation by high frequency currents. A receiving circuit 128 similar to that shown in Fig. 2 is employed for receiving the side band radiated by antenna 102. The unmodulated carrier component is supplied over circuit 121, including a transformer 129 which couples it to receiving circuit 128.

A dial 123 having an arm 133, adapted to make contact with points 124 permits the control of selector switch 118 by the subscriber. Control currents of 8000 cycles frequency may pass through selective device 120 by way of its switch arm 133 and contact points 124. Consequently by manipulation-of the dial the subscriber may operate his selector switch 118 to select any one of the trunks 108, 109 or 110 as desired and as a result to secure the program wanted.

Since it is likely that at certain times all of the trunks 108, 109 and 110 will be idle, it

is desirable that a positive indication be given to the subscriber so that he may know that the failure .of programs in his receiving set is due to no fault of his own apparatus but merely to absence of broadcasting programs. For this purpose a source 155 of two high frequencies differing in value from those maintained on trunks 108, 109 and 110 is provided at the central carrier current distributing exchange. This source is .associated with each of the trunks through normally open contacts of relays 135, 136 and 137 respectively. These relays respond to the action of vacuum tube detectors 138, 139 and 140 respectively.

Suppose, for example, that the program being broadcast over trunk 108 ceases; dctector 138 causes relay to release, closing its back contact. The source of the two high frequencies is thus connected to switch points 141 and 142. In similar fashion detectors 139 and 140 will cause relays 136- and 137 to release and to connect the source to the switch points of the other trunks. In order to give the subscriber a test indication, source 155 is connected over trunk 132 permanently to switch points 143 and 144. By operating his dial to .move his switch to the test position he can tell at any time whether his receiving set is functioning properly. The test is given by the interaction of the two high frequencies from source 155 which when impressed on the subscribers detector yield a component of their beat or difference frequency. This enables the subscriber to hear an audible note of the beat frequency.

To prevent the tying together of all the program channels through the common frequency channel of source 155, band pass filters 111, 112 and 113 are inserted between channels 105, 106 and 107 and trunks 108, 109 and 110, respectively, in order to prevent interaction between the channels and also to exclude currents from source 155 from the program channels.

Certain broad features disclosed in this application are the invention of James V.

Schmied and are claimed in his prior copending application Serial No. 689,627, filed January 31, 1924, entitled Carrier wave si al systems.

at is claimed is:

1. The method of secret radio signaling which comprises controlling the action of signal transmitting apparatus so as to produce two complementary components of a signal modulated wave, both of which are essential to intelligible reception of the signal, broadcasting one component through space and transmitting the other component by wire to selected receiving stations in the radio field of said one component.

2. The method of secret radio signaling which comprises controlling the action of signal transmitting apparatus so as to produce at least one signal modulated side band and an unmodulated carrier component, radiating said side bands and transmitting the unmodulated carrier component by wire to selected receiving stations in the field of the radiant energy transmission.

3. The method specified in claim 2 comprising the further step of continuously varying the carrier frequency.

4. The method of secret signaling which comprises generating oscillations of comparatively low frequency, raising the frequency of said oscillations to that carrier frequency required for efiicient modulation and radiation, modulating said carrier frequency wave in accordance with signals, radiating a portion of the resultant modulated wave which is adequate for intelligible reception when combined with a wave corresponding to said carrier frequency oscillations but otherwise incapable of intelligible reception, transmitting to desired receiving stations said low frequency oscillations, raising the frequency of the low frequency oscillations at said receiving stations to the frequency of said carrier wave, and combining the resultant carrier waves at the receiving stations with the received radiated wave to reproduce the signal.

5. The method of signaling which comprises producingat a given point a wave from which a wave'of carrier frequency can be derived and a signal side band of the carrier wave, radiating the side band to a distant point to the exclusion of said other waves, conducting the first mentioned wave to a point distant from said first mentioned point, deriving a wave of said carrier frequency from said wave so conducted, and combining said derived wave and said side band to reproduce said signal.

6. The method of secret signaling which comprises producing at a given point a wave from which a wave of carrier frequency can be derived and corresponding upper and lower signal side bands of the carrier wave, radiating one of said side bands to a distant point to the exclusion of said other waves and said other side band, conducting the first mentioned wave to a point distant from said first mentioned point, deriving a wave of said carrier frequenc from said wave so conducted, and combining said derived wave and said radiated side band to reproduce said signal.

In witness whereof, we hereunto subscribe our names this 3d of September, A. D. 1924.

' RAY R. ADAMS.

ELMER V. GRIGGS. 

